Objective: Recently we have shown that calpain-1 activation contributes to cardiomyocyte apoptosis induced by hyperglycemia. This study was undertaken to investigate whether targeted disruption of calpain would reduce myocardial hypertrophy and fibrosis in mouse models of type 1 diabetes.

Research design and methods: Diabetes in mice was induced by injection of streptozotocin (STZ), and OVE26 mice were also used as a type 1 diabetic model. The function of calpain was genetically manipulated by cardiomyocyte-specific knockout Capn4 in mice and the use of calpastatin transgenic mice. Myocardial hypertrophy and fibrosis were investigated 2 and 5 months after STZ injection or in OVE26 diabetic mice at the age of 5 months. Cultured isolated adult mouse cardiac fibroblast cells were also investigated under high glucose conditions.

Results: Calpain activity, cardiomyocyte cross-sectional areas, and myocardial collagen deposition were significantly increased in both STZ-induced and OVE26 diabetic hearts, and these were accompanied by elevated expression of hypertrophic and fibrotic collagen genes. Deficiency of Capn4 or overexpression of calpastatin reduced myocardial hypertrophy and fibrosis in both diabetic models, leading to the improvement of myocardial function. These effects were associated with a normalization of the nuclear factor of activated T-cell nuclear factor-κB and matrix metalloproteinase (MMP) activities in diabetic hearts. In cultured cardiac fibroblasts, high glucose-induced proliferation and MMP activities were prevented by calpain inhibition.

Figure 1: Calpain activity and protein expression in STZ-injected and OVE26 mice. Enzymatic calpain activity was measured in sham and STZ-treated hearts (A) or wild-type (WT) and OVE26 hearts (B). C: The cleavage of α-spectrin (120/150 KD) was determined in sham and STZ-injected hearts by Western blot analysis. Top: A representative Western blot for cleaved fragments of α-spectrin from four of eight different hearts in each group; bottom: quantification of cleaved fragments of α-spectrin relative to GAPDH. D: Representative Western blot for calpain-1, calpain-2, and GAPDH proteins from four of eight hearts in each group (sham and STZ-induced hearts). D1 and D2 are the quantification of calpain-1 and calpain-2 protein relative to GAPDH. Data are mean ± SD; n = 8. KD, kDa. *P < 0.05 vs. sham or WT.

Mentions:
To establish calpain as a protease of interest in understanding the mechanisms behind cardiomyopathy in the diabetic heart, we measured calpain activity in heart tissues from STZ-injected and OVE26 diabetic mice. Consistent with our recent report in short-term STZ-induced diabetes (14), both STZ and OVE26 hearts showed a significant increase in calpain activity over nondiabetic hearts (Fig. 1A and B). The activation of calpain was further confirmed by upregulation of cleaved α-spectrin in diabetic hearts (Fig. 1C). The upregulation of calpain activity was not as a result of alterations in the ratio of calpain to calpastatin expression because the protein and mRNA levels of calpain-1 and calpain-2 as well as calpastatin were not significantly altered in diabetic hearts (Fig. 1D and Supplementary Fig. 1). These results confirm the association between increased calpain activity and diabetes.

Figure 1: Calpain activity and protein expression in STZ-injected and OVE26 mice. Enzymatic calpain activity was measured in sham and STZ-treated hearts (A) or wild-type (WT) and OVE26 hearts (B). C: The cleavage of α-spectrin (120/150 KD) was determined in sham and STZ-injected hearts by Western blot analysis. Top: A representative Western blot for cleaved fragments of α-spectrin from four of eight different hearts in each group; bottom: quantification of cleaved fragments of α-spectrin relative to GAPDH. D: Representative Western blot for calpain-1, calpain-2, and GAPDH proteins from four of eight hearts in each group (sham and STZ-induced hearts). D1 and D2 are the quantification of calpain-1 and calpain-2 protein relative to GAPDH. Data are mean ± SD; n = 8. KD, kDa. *P < 0.05 vs. sham or WT.

Mentions:
To establish calpain as a protease of interest in understanding the mechanisms behind cardiomyopathy in the diabetic heart, we measured calpain activity in heart tissues from STZ-injected and OVE26 diabetic mice. Consistent with our recent report in short-term STZ-induced diabetes (14), both STZ and OVE26 hearts showed a significant increase in calpain activity over nondiabetic hearts (Fig. 1A and B). The activation of calpain was further confirmed by upregulation of cleaved α-spectrin in diabetic hearts (Fig. 1C). The upregulation of calpain activity was not as a result of alterations in the ratio of calpain to calpastatin expression because the protein and mRNA levels of calpain-1 and calpain-2 as well as calpastatin were not significantly altered in diabetic hearts (Fig. 1D and Supplementary Fig. 1). These results confirm the association between increased calpain activity and diabetes.

Bottom Line:
Calpain activity, cardiomyocyte cross-sectional areas, and myocardial collagen deposition were significantly increased in both STZ-induced and OVE26 diabetic hearts, and these were accompanied by elevated expression of hypertrophic and fibrotic collagen genes.These effects were associated with a normalization of the nuclear factor of activated T-cell nuclear factor-κB and matrix metalloproteinase (MMP) activities in diabetic hearts.Thus targeted inhibition of calpain represents a potential novel therapeutic strategy for reversing diabetic cardiomyopathy.

Objective: Recently we have shown that calpain-1 activation contributes to cardiomyocyte apoptosis induced by hyperglycemia. This study was undertaken to investigate whether targeted disruption of calpain would reduce myocardial hypertrophy and fibrosis in mouse models of type 1 diabetes.

Research design and methods: Diabetes in mice was induced by injection of streptozotocin (STZ), and OVE26 mice were also used as a type 1 diabetic model. The function of calpain was genetically manipulated by cardiomyocyte-specific knockout Capn4 in mice and the use of calpastatin transgenic mice. Myocardial hypertrophy and fibrosis were investigated 2 and 5 months after STZ injection or in OVE26 diabetic mice at the age of 5 months. Cultured isolated adult mouse cardiac fibroblast cells were also investigated under high glucose conditions.

Results: Calpain activity, cardiomyocyte cross-sectional areas, and myocardial collagen deposition were significantly increased in both STZ-induced and OVE26 diabetic hearts, and these were accompanied by elevated expression of hypertrophic and fibrotic collagen genes. Deficiency of Capn4 or overexpression of calpastatin reduced myocardial hypertrophy and fibrosis in both diabetic models, leading to the improvement of myocardial function. These effects were associated with a normalization of the nuclear factor of activated T-cell nuclear factor-κB and matrix metalloproteinase (MMP) activities in diabetic hearts. In cultured cardiac fibroblasts, high glucose-induced proliferation and MMP activities were prevented by calpain inhibition.